3,4-Dihydro-2,8-diisopropyl-3-thioxo-2H-1,4-benzoxazine-4-acetic acid (hereinafter referred to as the "present compound") is, for example, as disclosed in U.S. Pat. No. 4,771,050, a compound having excellent aldose reductase inhibitory activity and platelet aggregation inhibitory activity and, further, is useful as a medicine for preventing and treating diabetic complication such as diabetic cataract, keratopathy, retinopathy, neuropathy, nephropathy and the like. Thus, the present compound can be used as a medicine in the field of ophthalmology, for example, diabetic cataract, keratopathy, retinopathy and the like as described above. On the other hand, for using a medicine in the field of ophthalmology, generally, it is desirable to apply the medicine by eye drops rather than by oral administration or injection from the viewpoint of side effects. However, if the present compound is formulated as eye drops, there are problems of stability, ocular irritation and the like, and the problems have not yet been solved until now.
When the present compound is formulated as eye drops for the purpose of treating diabetic cataract, retinopathy and the like, there are unavoidable disadvantages that stability on storage for a long period of time is insufficient and an undesirable insoluble material is formed in the resulting eye drops. Further, strong ocular irritation is caused when the present compound is applied by eye drops in the form of an aqueous solution. Therefore, although the present compound has excellent pharmacological activities, any practical eye drops containing it can not be obtained.